1. Field of the Invention
This invention relates to a method for the surface treatment of an absorbent resin. More particularly, it relates to a method for the surface treatment of an absorbent resin, which method is ideal for the production of an absorbent agent exhibiting excellent physical properties such as a high capacity for absorption under load, excels in safety, and hydroscopic flowability.
2. Description of the Prior Art
In recent years, absorbent resins which absorb a large volume of water and consequently take on the form of gel have been developed and have been finding extensive utility in the field of agriculture and forestry and the field of civil engineering as well as in the field of sanitary materials such as disposable diapers and sanitary napkins.
The absorbent resins heretofore known in the art include cross-linked polyacrylic salts, hydrolyzed starch-acrylonitrile graft polymer, starch-acrylic acid (acrylic salt) graft polymer, saponified cross-linked vinyl acetate-acrylate copolymers, cross-linked polyacrylamide, cross-linked polymer of 2-acrylamide-2-methyl propanesulfonic acid (propanesulfonic salt), hydrolyzed cross-linked polyacrylamide, and cross-linked cationic monomers, for example.
These absorbent resins are insolubilized hydrophilic resins which have a uniform cross-linked structure in their polymeric units. Generally, the particles of these absorbent resins have their surfaces additionally cross-linked as with a cross-linking agent to impart a cross-link density gradient to the particles and enable the absorbent resins to acquire improved physical properties as evinced by addition to the absorption rate of water, preclusion of the formation of wet clusters of particulate resin, exaltation of gel strength, improvement in the absorption capacity of water under load, prevention of the phenomenon of gel blocking, and enhancement of the liquid permeability. In the case of the absorbent resins which have such functional groups as, for example, a carboxyl group or a hydroxyl group, since the surface treatment mentioned above affects the basic physical properties of the absorbent resins to a large extent, numerous cross-linking agents and many methods for their mixing have been proposed to date.
The methods heretofore proposed for the mixing of cross-linking agents are broadly divided into those comprising dispersing an absorbent resin in a solvent (JP-A-56-131,608, JP-A-57-44,627, JP-A-58-42,602, and JP-A-58-117,222) and those comprising directly adding a treating agent or a solution of the treating agent to an absorbent resin. In the case of the latter methods which comprise direct addition of a cross-linking agent, many methods have been proposed for ensuring uniform addition of a cross-linking agent and water. These methods include a method which relies on the use of an inert inorganic powder (JP-A-60-163,956 and JP-A-60-255,814), a method which comprises first adding a cross-linking agent and subsequently exposing it to steam (JP-A-1-113,406), a method which resides in heating a cross-linking agent under specific humidity conditions (JP-A-1-297,430), a method which resorts to the use of a mixed solvent comprising water and a polyhydric alcohol (JP-A-63-270,741, JP-A-64-56,707, and JP-A-1-292,004), and a method which involves the use of a mixed solvent consisting of water and an ether compound (JP-A-2-153,903), for example.
As typical examples of the cross-linking agents proposed to date, polyglycidyl compounds, haloepoxy compounds, polyisocyanate compounds, polyaziridine compounds (JP-A-59-189,103), polyoxazoline compounds, polyamine compounds, polyhydric alcohols (JP-A-58-180,233 and JP-A-61-16,903), alkylene carbonates (DE-A-4020780C), glyoxals (JP-A-52-117,393), polyvalent metal salts (U.S. Pat. No. 4,043,952, JP-A-61-257,235, JP-A-62-7,745), silane coupling agents (JP-A-61-211,305, JP-A-61-252,212, and JP-A-61-264,006), peroxide radical initiators (JP-A-63-99,211), and special glycidyl compounds of specific structures (JP-A-62-50,305, JP-A-61-213,206, JP-A-63-199,205, JP-A-63-118,308, JP-A-487,638, JP-A-1-201,312, and JP-A-61-293,228) may be cited.
These cross-linking agents are broadly divided into (1) ring-opening reactive cross-linking agents such as glycidyl compounds, (2) condensation reactive cross-linking agents such as isocyanate compounds, (3) ion-bonding crosslinking agents such as polyvalent metals, and (4) dehydration reactive cross-linking agents such as polyhydric alcohols.
In the four kinds of cross-linking agents mentioned above, the ion-bonding cross-linking agents of the kind of (3) do not withstand actual use because they are deficient in the binding force with an absorbent resin. The cross-linking agents of the kinds of (1) to (3), because of their high reactivity, react immediately with an absorbent resin on contact therewith and alter the physical properties of the absorbent resin and, therefore, encounter difficulty in being uniformly distributed in the surface region of the absorbent resin over a long duration following the time of mixture. In spite of the numerous methods proposed for the addition of a cross-linking agent as described above, it is difficult to attain uniform cross-linkage of an absorbent resin with the cross-linking agents of the kinds of (1) to (3) and permit impartation of ideal physical properties to the absorbent resin. Generally, when a cross-linking agent is used in cross-linking an absorbent resin, part of the cross-linking agent remains in an unaltered form in the absorbent resin without reference to the kind of the crosslinking agent. The fact that such a cross-linking agent as a glycidyl compound which has high reactivity remains, if partly, in the unaltered form on the surface of an absorbent resin is undesirable from the standpoint of safety. A method for decreasing the amount of the remaining cross-linking agent, therefore, has been proposed (JP-A-3-195,705). This method is at a disadvantage in entailing a complicated process and, moreover, attaining the expected decrease only with great difficulty.
Unlike the cross-linking agents of the kinds of (1) to (3), such dehydration reactive cross-linking agents of the kind of (4) as polyhydric alcohols are free from the disadvantage of reacting with an absorbent resin immediately after their contact with the absorbent resin. They are, therefore, distributed gradually and uniformly on the surface of the absorbent resin and allowed to effect a uniform surface treatment easily on the absorbent resin. Polyhydric alcohols generally have high stability and safety and, therefore, pose no problem if they happen to persist on the surface of an absorbent resin. Thus, polyhydric alcohols have been used particularly preferably as a cross-linking agent in the surface treatment of an absorbent resin from the standpoint of physical properties and safety.
Since cross-linking agents such as of polyhydric alcohols generally exhibit extremely high degrees of hydrophilicity and hygroscopicity, they possibly permeate an absorbent resin to a great depth at a sacrifice of the effect of surface treatment, depending on the conditions of mixture and the reaction conditions. When these cross-linking agents of polyhydric alcohols remain on the surface of the absorbent resin, they exalt the hygroscopicity and adhesiveness of the treated absorbent resin after absorbing moisture excessively. When the absorbent resin resulting from this surface treatment is put to actual use in a factory full of moisture as in a rainy season, therefore, it entails the disadvantage that the particulate mass of the absorbent resin agglomerate and induce the phenomenon of blocking.
For the purpose of preventing the absorbent resin from this mischief of blocking, a method which resorts to addition of a fine inorganic powder having an approximate particle diameter of several microns to the absorbent resin has been proposed (JP-A-59-80,459). This method, however, has the disadvantage that the fine inorganic powder is extremely expensive and, on being drifted as in wind, is liable to imperil the worker's health and impair the plant's operational efficiency.
An object of this invention, therefore, is to provide a method for the surface treatment of an absorbent resin.
Another object of this invention is to utilize the surface treatment for producing an absorbent resin which (1) excels in safety, (2) manifests an effect in ensuring uniform mixture with a cross-linking agent and consequently improving the physical properties of the absorbent resin, and (3) avoids inducing agglomeration of the particulate mass of the absorbent resin even in a highly humid atmosphere.